Semiconductor device

ABSTRACT

A semiconductor device includes: a first electrode; a first semiconductor layer of first conductivity type provided on the first electrode; a second semiconductor layer of first conductivity type provided on the first semiconductor layer; a first semiconductor region of second conductivity type provided on the second semiconductor layer; a second semiconductor region of first conductivity type provided on the first semiconductor region; a first insulating film provided in a trench reaching the second semiconductor layer from above the second semiconductor region via the second semiconductor region and the first semiconductor region, the first insulating film containing a first insulating material; a second electrode provided in the trench, the second electrode facing the second semiconductor layer via the first insulating film; a second insulating film provided between a position of 40% of a height of the second electrode from a lower end of the second electrode and a position of an upper end of the second electrode, the second insulating film being provided between the side surface of the second electrode and a fifth insulating film provided between a side surface of the second electrode and the second semiconductor layer, the fifth insulating film containing the first insulating material, the second insulating film containing a second insulating material having a higher dielectric constant than the first insulating material; a third electrode provided above the second electrode, the first insulating film and the second insulating film, the third electrode facing the first semiconductor region via a gate insulating film; an interlayer insulating film provided on the third electrode; and a fourth electrode provided above the interlayer insulating film, wherein the first insulating film in the trench below the position of 40% of the height of the second electrode contains only the first insulating material.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2020-046909, filed on Mar. 17, 2020, theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a semiconductor device.

BACKGROUND

A semiconductor device such as a metal oxide semiconductor field effecttransistor (MOSFET) is used for power conversion and the like. Such asemiconductor device desirably has a high breakdown voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a semiconductor deviceaccording to a first embodiment;

FIG. 2 is a schematic cross-sectional view of a main part of thesemiconductor device according to the first embodiment;

FIG. 3 is a schematic cross-sectional view of a main part of thesemiconductor device according to the first embodiment;

FIG. 4 is a schematic cross-sectional view of a semiconductor deviceaccording to another aspect of the first embodiment;

FIG. 5 is a schematic cross-sectional view of a main part of asemiconductor device according to another aspect of the firstembodiment;

FIG. 6 is a schematic cross-sectional view illustrating a process ofmanufacturing the semiconductor device according to the firstembodiment;

FIG. 7 is a schematic cross-sectional view illustrating the process ofmanufacturing the semiconductor device according to the firstembodiment;

FIG. 8 is a schematic cross-sectional view illustrating the process ofmanufacturing the semiconductor device according to the firstembodiment;

FIG. 9 is a schematic cross-sectional view illustrating the process ofmanufacturing the semiconductor device according to the firstembodiment;

FIG. 10 is a schematic cross-sectional view illustrating the process ofmanufacturing the semiconductor device according to the firstembodiment;

FIG. 11 is a schematic cross-sectional view illustrating the process ofmanufacturing the semiconductor device according to the firstembodiment;

FIG. 12 is a schematic cross-sectional view illustrating the process ofmanufacturing the semiconductor device according to the firstembodiment;

FIG. 13 is a schematic cross-sectional view illustrating the process ofmanufacturing the semiconductor device according to the firstembodiment;

FIG. 14 is a schematic cross-sectional view illustrating the process ofmanufacturing the semiconductor device according to the firstembodiment;

FIGS. 15A to 15C are schematic diagrams illustrating a function and aneffect of the semiconductor device according to the first embodiment;and

FIG. 16 is a schematic cross-sectional view of a main part of asemiconductor device according to a second embodiment.

DETAILED DESCRIPTION

Hereinafter, embodiments of the present disclosure will be describedwith reference to the drawings. Note that in the following description,the same members and the like are denoted by the same referencenumerals, and description of members and the like once described isappropriately omitted.

Here, upward in the drawings is described as “up”, and downward in thedrawings is described as “down” in order to indicate a positionalrelationship of parts and the like. Here, the terms “up” and “down” donot necessarily indicate a relationship with the direction of gravity.

Hereinafter, a case where a first conductivity type is n-type and asecond conductivity type is p-type will be exemplified.

In the following description, notations of n⁺, n, n⁻, p⁺, p, and p⁻indicate a relative level of an impurity concentration of each of theconductivity types. That is, n⁺ indicates that an impurity concentrationof n-type is relatively higher than n, and n⁻ indicates that theimpurity concentration of n-type is relatively lower than n. p⁺indicates that an impurity concentration of p-type is relatively higherthan p, and p⁻ indicates that the impurity concentration of p-type isrelatively lower than p. Note that n⁺ type and n⁻ type may be simplyreferred to as n type, and p⁺ type and p⁻ type may be simply referred toas p type.

First Embodiment

A semiconductor device according to the present embodiment includes: afirst electrode; a first semiconductor layer of first conductivity typeprovided on the first electrode; a second semiconductor layer of firstconductivity type provided on the first semiconductor layer; a firstsemiconductor region of second conductivity type provided on the secondsemiconductor layer; a second semiconductor region of first conductivitytype provided on the first semiconductor region; a first insulating filmprovided in a trench reaching the second semiconductor layer from abovethe second semiconductor region via the second semiconductor region andthe first semiconductor region, the first insulating film containing afirst insulating material; a second electrode provided in the trench,the second electrode facing the second semiconductor layer via the firstinsulating film; a second insulating film provided between a position of40% of a height of the second electrode from a lower end of the secondelectrode and a position of an upper end of the second electrode, thesecond insulating film being provided between the side surface of thesecond electrode and a fifth insulating film provided between a sidesurface of the second electrode and the second semiconductor layer, thefifth insulating film containing the first insulating material, thesecond insulating film containing a second insulating material having ahigher dielectric constant than the first insulating material; a thirdelectrode provided above the second electrode, the first insulating filmand the second insulating film, the third electrode facing the firstsemiconductor region via a gate insulating film; an interlayerinsulating film provided on the third electrode; and a fourth electrodeprovided above the interlayer insulating film, wherein the firstinsulating film in the trench below the position of 40% of the height ofthe second electrode contains only the first insulating material.

In addition, the semiconductor device according to the presentembodiment includes: a first electrode; a first semiconductor layer offirst conductivity type provided on the first electrode; a secondsemiconductor layer of first conductivity type provided on the firstsemiconductor layer; a first semiconductor region of second conductivitytype provided on the second semiconductor layer; a second semiconductorregion of first conductivity type provided on the first semiconductorregion; a first insulating film provided in a trench reaching the secondsemiconductor layer from above the second semiconductor region via thesecond semiconductor region and the first semiconductor region, thefirst insulating film containing a first insulating material; a secondelectrode provided in the trench, the second electrode facing the secondsemiconductor layer via the first insulating film; a second insulatingfilm provided between a position of 40% of a height of the secondelectrode from a lower end of the second electrode and a position of anupper end of the second electrode, the second insulating film beingprovided between the side surface of the second electrode and a fifthinsulating film provided between a side surface of the second electrodeand the second semiconductor layer, the fifth insulating film containingthe first insulating material, the second insulating film containing asecond insulating material having a higher dielectric constant than thefirst insulating material; a third electrode provided above the secondelectrode, the first insulating film and the second insulating film, thethird electrode facing the first semiconductor region via a gateinsulating film; an interlayer insulating film provided on the thirdelectrode; and a fourth electrode provided above the interlayerinsulating film, wherein the first insulating film in the trench belowthe position of 40% of the height of the second electrode does notcontains the second insulating material.

FIG. 1 is a schematic cross-sectional view of a semiconductor device 100according to the present embodiment. FIG. 2 is a schematiccross-sectional view of a main part of the semiconductor device 100according to the present embodiment. FIG. 2 is a view for explaining afirst trench 20. FIG. 3 is a schematic view of a main part of thesemiconductor device 100 according to the present embodiment. FIG. 3 isa view for explaining a second trench 40. The semiconductor device 100is, for example, a vertical MOSFET.

The semiconductor device 100 includes a drain layer 10, a drift layer12, a base region 14, a source region 16, the first trench 20, a firstinsulating film 22, a field plate electrode 24, a second insulating film26, a third insulating film 30, a fourth insulating film 32, a fifthinsulating film 34, a sixth insulating film 36, a gate insulating film37, the second trench 40, a seventh insulating film 42, a field plateelectrode 44, an eighth insulating film 46, a gate insulating film 47, aninth insulating film 50, a tenth insulating film 52, an eleventhinsulating film 54, a twelfth insulating film 56, a drain electrode 60,a gate electrode 62, a barrier metal 64, a source electrode 66, and aninterlayer insulating film 70.

Note that the drain electrode 60 is an example of the first electrode.The drain layer 10 is an example of the first semiconductor layer. Thedrift layer 12 is an example of the second semiconductor layer. The baseregion 14 is an example of the first semiconductor region. The sourceregion 16 is an example of the second semiconductor region. The firsttrench 20 is an example of the trench. The field plate electrode 24 isan example of the second electrode. The gate electrode 62 is an exampleof the third electrode. The source electrode 66 is an example of thefourth electrode.

The drain layer 10 functions as a drain of the MOSFET. The drain layer10 contains, for example, a semiconductor material of n⁺ type.

The drain electrode 60 is provided under the drain layer 10 and iselectrically connected to the drain layer 10. In other words, the drainlayer 10 is provided on the drain electrode 60. The drain electrode 60functions as a drain electrode of the MOSFET.

The drift layer 12 is provided on the drain layer 10. The drift layer 12functions as a drift layer of the MOSFET. The drift layer 12 contains,for example, a semiconductor material of n⁻ type.

The base region 14 is provided on the drift layer 12. The base region 14functions as a base of the MOSFET. The base region 14 forms a channeland allows carriers to flow between the source region 16 and the drainlayer 10 when a voltage is applied to the gate electrode 62. The baseregion 14 contains, for example, a p-type semiconductor material. Thesemiconductor device 100 includes base regions 14 a, 14 b, and 14 c asthe base region 14.

The source region 16 is provided on the base region 14. The sourceregion 16 functions as a source of the MOSFET. When an appropriatevoltage is applied to the gate electrode 62, carriers flow between thesource region 16 and the drain layer 10. The source region 16 contains,for example, a semiconductor material of n⁺ type. The semiconductordevice 100 includes source regions 16 a, 16 b, 16 c, and 16 d as thesource region 16.

Here, an X direction, a Y direction perpendicular to the X direction,and a Z direction perpendicular to the X direction and the Y directionare defined. The drain layer 10 and the drift layer 12 are providedparallel to an XY plane parallel to the X direction and the Y direction(horizontal direction). The Z direction is a direction in which thedrain layer 10 and the drift layer 12 are stacked.

The first trench 20 is provided so as to reach the drift layer 12 fromabove the source region 16 via the source region 16 and the base region14.

The first insulating film 22 is provided in the first trench 20. Thefirst insulating film 22 contains the first insulating material. Thefirst insulating material is, for example, silicon oxide (SiOx), but isnot limited to SiOx.

The field plate electrode 24 is provided in the first trench 20 so as toface the drift layer 12 via the first insulating film 22. For example,the field plate electrode 24 is provided alongside the drift layer 12.The field plate electrode 24 is provided, for example, in order toreduce concentration of a reverse electric field between the sourceelectrode 66 and the drain electrode 60 to increase a breakdown voltage.The field plate electrode 24 is provided so as to extend in the Zdirection. The field plate electrode 24 has, for example, a portionextending upward in a portion (not illustrated) provided in thedirection extending into the page of FIG. 1. The field plate electrode24 is electrically connected to the source electrode 66 using theupwardly extending portion. Note that how to connect the field plateelectrode 24 to the source electrode 66 is not limited to thisconnecting method.

A second insulating film 26 a as the second insulating film 26 isprovided in the first trench 20 between a side surface 24 c of the fieldplate electrode and the fifth insulating film 34. The second insulatingfilm 26 a is provided between the position of 40% of the height of thefield plate electrode 24 from a lower end 24 a of the field plateelectrode and the position of an upper end 24 b of the field plateelectrode. The “position of 40% of the height of the field plateelectrode 24 from the lower end 24 a of the field plate electrode” willbe described. A distance between an XY plane P₁ passing through thelower end 24 a of the field plate electrode and an XY plane P₃ passingthrough the upper end 24 b of the field plate electrode is referred toas L₁. Next, consider such an XY plane P₂ that a distance L₂ from the XYplane P₁ passing through the lower end 24 a of the field plate electrodesatisfies L₂=0.4L₁, that is, L₂ is 40% of L₁. The position of this XYplane P₂ is the “position of 40% of the height of the field plateelectrode 24 from the lower end 24 a of the field plate electrode”. Inother words, the second insulating film 26 a is provided between the XYplane P₂ and the XY plane P₃. Note that the second insulating film 26 amay be further provided above the XY plane P₃. FIG. 2 illustrates a casewhere the second insulating film 26 a is provided above the XY plane P₃.

Similarly, a second insulating film 26 b as the second insulating film26 is provided in the first trench 20 between a side surface 24 d of thefield plate electrode and the sixth insulating film 36. The secondinsulating film 26 b is provided between the position of 40% of theheight of the field plate electrode 24 from a lower end 24 a of thefield plate electrode and the position of an upper end 24 b of the fieldplate electrode.

The second insulating film 26 a may have a first portion 26 a ₁ and asecond portion 26 a ₂ provided on the first portion 26 a ₁. In thiscase, the second portion 26 a ₂ has a larger film thickness than thefirst portion 26 a ₁. Here, the film thickness is, for example, a lengthin a plane parallel to the XY plane (in the horizontal plane).

Similarly, the second insulating film 26 b may have a third portion 26 b₁ and a fourth portion 26 b ₂ provided on the third portion 26 b ₁. Inthis case, the fourth portion 26 b ₂ has a larger film thickness thanthe third portion 26 b ₁. Here, the film thickness is, for example, alength in a plane parallel to the XY plane (in the horizontal plane).

The film thickness W₂ of the second insulating film 26 between the sidesurface 24 c of the field plate electrode and the drift layer 12 ispreferably 30% or more of the film thickness W₁ of the first insulatingfilm 22 between the side surface 24 c of the field plate electrode andthe drift layer 12, where the second insulating film 26 is not provided.Here, the film thickness is, for example, a length in a plane parallelto the XY plane.

The second insulating film 26 contains the second insulating materialhaving a higher dielectric constant than the first insulating material.The second insulating material is, for example, silicon nitride (SiNx),but is not limited to SiNx.

The third insulating film 30 is provided in the first trench 20 betweenthe side surface 24 c of the field plate electrode and the secondinsulating film 26 a. The third insulating film 30 contains the firstinsulating material.

The fourth insulating film 32 is provided in the first trench 20 betweenthe side surface 24 d of the field plate electrode and the secondinsulating film 26 b. The fourth insulating film 32 contains the firstinsulating material.

The fifth insulating film 34 is provided in the first trench 20 betweenthe drift layer 12 and the second insulating film 26 a. The fifthinsulating film 34 contains the first insulating material.

The sixth insulating film 36 is provided in the first trench 20 betweenthe drift layer 12 and the second insulating film 26 b. The sixthinsulating film 36 contains the first insulating material.

A first gate electrode 62 a as the gate electrode 62 is provided abovethe field plate electrode 24 so as to face the base region 14 via thegate insulating film 37. The first gate electrode 62 a functions as agate of the MOSFET. The gate insulating film 37 contains, for example,the first insulating material.

For example, the first insulating film 22, the third insulating film 30,the fourth insulating film 32, the fifth insulating film 34, the sixthinsulating film 36, and the gate insulating film 37 may be formedsimultaneously in the same step, or may be formed in different steps.

The second trench 40 is provided so as to reach the drift layer 12 fromabove the source region 16 via the source region 16 and the base region14.

The seventh insulating film 42 is provided in the second trench 40. Theseventh insulating film 42 contains the first insulating material.

The field plate electrode 44 is provided in the second trench 40 so asto face the drift layer 12 via the seventh insulating film 42. Forexample, the field plate electrode 44 is provided alongside the driftlayer 12. The field plate electrode 44 is provided, for example, inorder to reduce concentration of a reverse electric field between thesource electrode 66 and the drain electrode 60 to increase a breakdownvoltage. The field plate electrode 44 is provided so as to extend in theZ direction. The field plate electrode 44 has, for example, a portionextending upward in a portion (not illustrated) provided in thedirection extending into the page of FIG. 1. The field plate electrode44 is electrically connected to the source electrode 66 using theupwardly extending portion. Note that how to connect the field plateelectrode 44 to the source electrode 66 is not limited to thisconnecting method.

An eighth insulating film 46 a as the eighth insulating film 46 isprovided in the second trench 40 between a side surface 44 c of thefield plate electrode and the eleventh insulating film 54. The eighthinsulating film 46 a is provided between the position of 40% of theheight of the field plate electrode 44 from a lower end 44 a of thefield plate electrode and the position of an upper end 44 b of the fieldplate electrode. In other words, the eighth insulating film 46 a isprovided between the XY plane P₂ and the XY plane P₃. Note that theeighth insulating film 46 a may be further provided above the XY planeP₃. FIG. 3 illustrates a case where the eighth insulating film 46 a isprovided above the XY plane P₃.

Similarly, an eighth insulating film 46 b as the eighth insulating film46 is provided in the second trench 40 between a side surface 44 d ofthe field plate electrode and the twelfth insulating film 56. The eighthinsulating film 46 b is provided between the position of 40% of theheight of the field plate electrode 44 from a lower end 44 a of thefield plate electrode and the position of an upper end 44 b of the fieldplate electrode.

The ninth insulating film 50 is provided in the second trench 40 betweenthe side surface 44 c of the field plate electrode and the eighthinsulating film 46 a. The ninth insulating film 50 contains the firstinsulating material.

The tenth insulating film 52 is provided in the second trench 40 betweenthe side surface 44 d of the field plate electrode and the eighthinsulating film 46 b. The tenth insulating film 52 contains the firstinsulating material.

The eleventh insulating film 54 is provided in the second trench 40between the drift layer 12 and the eighth insulating film 46 a. Theeleventh insulating film 54 contains the first insulating material.

The twelfth insulating film 56 is provided in the second trench 40between the drift layer 12 and the eighth insulating film 46 b. Thetwelfth insulating film 56 contains the first insulating material.

A second gate electrode 62 b as the gate electrode 62 is provided abovethe field plate electrode 44 so as to face the base region 14 via thegate insulating film 47. The second gate electrode 62 b functions as agate of the MOSFET. The gate insulating film 47 contains, for example,the first insulating material.

For example, the seventh insulating film 42, the ninth insulating film50, the tenth insulating film 52, the eleventh insulating film 54, thetwelfth insulating film 56, and the gate insulating film 47 may beformed simultaneously in the same step, or may be formed in differentsteps.

An interlayer insulating film 70 a as the interlayer insulating film 70is provided on the first gate electrode 62 a, the source region 16 a,and the source region 16 b. An interlayer insulating film 70 b as theinterlayer insulating film 70 is provided on the second gate electrode62 b, the source region 16 c, and the source region 16 d.

The source electrode 66 is provided above the base region 14, the sourceregion 16, and the interlayer insulating film 70. The source electrode66 functions as a source of the MOSFET.

The barrier metal 64 is provided between the source electrode 66 and thebase region 14, the source region 16, and the interlayer insulating film70. The barrier metal 64 is a film used for preventing a reactionbetween the source electrode 66 and a semiconductor material used forthe semiconductor device 100. The barrier metal 64 contains, forexample, titanium (Ti), titanium nitride (TiN), tantalum (Ta), ortantalum nitride (TaN).

FIG. 4 is a schematic cross-sectional view of a semiconductor device 110according to another aspect of the present embodiment. FIG. 5 is aschematic cross-sectional view of a main part of the semiconductordevice 110 according to another aspect of the first embodiment. Adifference does not have to be made in the film thickness in the up-downdirection between the second insulating film 26 a and the secondinsulating film 26 b. Similarly, a difference does not have to be madein the film thickness in the up-down direction between the eighthinsulating film 46 a and the eighth insulating film 46 b.

Examples of a semiconductor material used for the drain layer 10, thedrift layer 12, the base region 14, and the source region 16 includesilicon (Si). However, the semiconductor material used for the drainlayer 10, the drift layer 12, the base region 14, and the source region16 may be another semiconductor material such as silicon carbide (SiC),gallium nitride (GaN), or gallium arsenide (GaAs).

When silicon is used as the semiconductor material, for example, arsenic(As), phosphorus (P), or antimony (Sb) can be used as n-type impurities,and boron (B) can be used as p-type impurities.

The gate electrode 62, the field plate electrode 24, and the field plateelectrode 44 each contain a conductive material such as polysiliconcontaining impurities.

The drain electrode 60 and the source electrode 66 each contain a metalsuch as aluminum (Al).

FIGS. 6 to 14 are schematic cross-sectional views illustrating a processof manufacturing the semiconductor device 100 according to the presentembodiment.

First, the drift layer 12 and the base region 14 are formed on the drainlayer 10. For example, the drain layer 10 is used as a semiconductorsubstrate which is a Si substrate, and the drift layer 12 is formed onthe drain layer 10 by epitaxial growth. However, the process ofmanufacturing the drain layer 10 and the drift layer 12 is not limitedto the above manufacturing process. Next, the insulating film 80containing the first insulating material which is, for example, siliconoxide is formed on the drift layer 12 by, for example, a thermaloxidation method or chemical vapor deposition (CVD) (FIG. 6). Next, apart of the insulating film 80 is removed by, for example,photolithography and reactive ion etching (RIE) (FIG. 7). Next, usingthe insulating film 80 as a mask, trenches 82 and 84 reaching the driftlayer 12 from above the source region 16 are formed (FIG. 8). Next, thesixth insulating film 36 and the twelfth insulating film 56 eachcontaining the first insulating material which is, for example, siliconoxide are formed in the trench 82 and the trench 84 on the drift layer12 by a thermal oxidation method or CVD, respectively (FIG. 9). Next, aninsulating film 90 containing the second insulating material which is,for example, silicon nitride is formed on the sixth insulating film 36and the twelfth insulating film 56 by, for example, low pressurechemical vapor deposition (LPCVD) (FIG. 10).

Next, a part of the insulating film 90 formed on the sixth insulatingfilm 36 and the twelfth insulating film 56, a part of the sixthinsulating film 36, and a part of the twelfth insulating film 56 at abottom of the trench 82 and a bottom of the trench 84 are removed by,for example, photolithography and RIE. Next, trenches 86 and 88extending below the trenches 82 and 84 are formed by, for example, RIEwhich is anisotropic etching and chemical dry etching (CDE) which isisotropic etching. By removing the part of the sixth insulating film 36in the trench 82, the sixth insulating film 36 becomes the fifthinsulating film 34 and the sixth insulating film 36. By removing thepart of the twelfth insulating film 56 in the trench 84, the twelfthinsulating film 56 becomes the eleventh insulating film 54 and thetwelfth insulating film 56 (FIG. 11).

Next, an inner wall of the trench 86 and an inner wall of the trench 88below the insulating film 90 are thermally oxidized by, for example,local oxidation of silicon (LOCOS). This forms the first trench 20having the first insulating film 22 containing the first insulatingmaterial which is silicon oxide provided inside and the second trench 40having the seventh insulating film 42 containing the first insulatingmaterial which is silicon oxide provided inside. Note that, for example,the third insulating film 30, the fourth insulating film 32, the ninthinsulating film 50, and the tenth insulating film 52 each containing thefirst insulating material are also formed around the insulating film 90.However, the oxidation rate of silicon nitride is lower than that of Si.Therefore, the third insulating film 30 and the fourth insulating film32 each have a smaller film thickness than the first insulating film 22,and the ninth insulating film 50 and the tenth insulating film 52 eachhave a smaller film thickness than the seventh insulating film 42. Holes92 and 94 are left inside and above the first insulating film 22 andinside and above the seventh insulating film 42 (FIG. 12). The thirdinsulating film 30 and the fourth insulating film 32, the ninthinsulating film 50 and the tenth insulating film 52, and the firstinsulating film 22 and the seventh insulating film 42 may be formed onlyby the LOCOS. After the LOCOS is performed, a film containing the firstinsulating material may be additionally stacked by, for example, the CVDmethod.

Next, the field plate electrode 24 and the field plate electrode 44 eachcontaining polysilicon containing impurities are formed inside the hole92 and inside the hole 94, respectively, for example, by CVD (FIG. 13).

Next, a part of each of the third insulating film 30, the fourthinsulating film 32, the insulating film 90, and the insulating film 80provided above the field plate electrode 24 and the field plateelectrode 44 is removed by etching and CDE. The insulating film 90 inthe first trench 20 becomes the second insulating film 26 a and thesecond insulating film 26 b which are the second insulating films 26.The insulating film 90 in the second trench 40 becomes the eighthinsulating film 46 a and the eighth insulating film 46 b which are theeighth insulating films 46. The insulating film 80 between the driftlayer 12 and the second insulating film 26 a becomes the fifthinsulating film 34. The insulating film 80 between the drift layer 12and the second insulating film 26 b becomes the sixth insulating film36. The insulating film 80 between the drift layer 12 and the eighthinsulating film 46 a becomes the eleventh insulating film 54. Theinsulating film 80 between the drift layer 12 and the eighth insulatingfilm 46 b becomes the twelfth insulating film 56 (FIG. 14).

Next, the gate electrode 62, the base region 14, the source region 16,the gate insulating film 37, the gate insulating film 47, the interlayerinsulating film 70, the barrier metal 64, the source electrode 66, andthe drain electrode 60 are formed to obtain the semiconductor device 100according to the present embodiment.

Next, a function and an effect of the present embodiment will bedescribed.

In the MOSFET having an up-down electrode structure as in the presentembodiment, the impurity concentration and the film thickness of thedrift layer 12 are adjusted within a predetermined range in order tomaintain an element breakdown voltage during switching off. The impurityconcentration and the film thickness of the drift layer 12 are limitedby a physical property limit of a semiconductor material forming thedrift layer 12. Therefore, there is a trade-off relationship between anelement breakdown voltage and an on-resistance.

There is a MOSFET in which the field plate electrode 24 electricallyconnected to the source electrode 66 or the gate electrode 62 isprovided below the trench-type gate electrode 62. By providing the fieldplate electrode 24 below the gate electrode 62, a depletion layerspreads between the trenches when a voltage is applied to the drainelectrode 60. This makes it possible to increase the impurityconcentration of the drift layer 12 without a decrease in the elementbreakdown voltage. As a result, an on-resistance can be decreased in theMOSFET including the field plate electrode 24.

Meanwhile, in the MOSFET including the field plate electrode 24, forexample, in a case where a breakdown voltage of 200 V or more isrequired, when it is tried to deepen a trench and to increase the filmthickness of an insulating film formed in the trench in order to obtaina desired breakdown voltage, a semiconductor substrate (wafer) on whichthe MOSFET is formed may be excessively stressed, and the wafer may bewarped, which makes manufacturing difficult disadvantageously. When thedepth of a trench and the film thickness of an insulating film formed inthe trench are suppressed, and instead, the n-type impurityconcentration of a part of the drift layer 12 is lowered to increase abreakdown voltage in order to avoid this problem, an on-resistanceincreases disadvantageously. In a case where a so-called super junctionstructure is used, in a p-type impurity-containing layer and an n-typeimpurity-containing layer provided alternately, it is more difficult tocontrol the p-type impurity content and the n-type impurity content asthe impurity concentrations of the p-type impurity-containing layer andthe n-type impurity-containing layer increase. Therefore, anon-resistance is high in a specific breakdown voltage regiondisadvantageously as compared with the MOSFET including the field plateelectrode 24 because it is difficult to increase the impurityconcentration.

FIGS. 15A to 15C are schematic diagrams for explaining a function and aneffect of the semiconductor device 100 according to the presentembodiment. FIGS. 15A to 15C are schematic diagrams illustrating anelectric field distribution in a depth direction of the semiconductordevice. FIG. 15A illustrates an ideal rectangular electric fielddistribution. FIG. 15B is a schematic diagram when an insulating film ina trench has a uniform film thickness. In this case, since the electricfield distribution in the depth direction of the semiconductor device isnon-uniform, a breakdown voltage decreases.

The semiconductor device 100 according to the present embodimentincludes the second insulating film 26 provided between the position of40% of the height of the field plate electrode 24 from the lower end 24a of the field plate electrode and the position of the upper end 24 b ofthe field plate electrode between the side surface 24 c of the fieldplate electrode and the first insulating film 22, the second insulatingfilm 26 containing the second insulating material having a higherdielectric constant than the first insulating material.

For example, an electric field distribution in the depth direction ofthe semiconductor device 100 according to the present embodiment isschematically illustrated as in FIG. 15C. This makes it possible to makethe effective film thickness of an insulating film in an upper portionof the first trench 20 smaller than the effective film thickness of theinsulating film in a lower portion of the first trench 20. This makes itpossible to make the electric field distribution in the semiconductordevice 100 closer to a rectangular shape when a reverse voltage isapplied. Therefore, a semiconductor device having a high breakdownvoltage can be provided. In addition, it is not necessary to separatelyprovide a layer having a lower n-type impurity concentration at a bottomof the drift layer 12. Therefore, it is possible to provide asemiconductor device having a high breakdown voltage while an increasein on-resistance is suppressed.

In the semiconductor device 100 according to the present embodiment, thefirst insulating film 22 in the first trench 20 below the position of40% of the height of the field plate electrode 24 contains only thefirst insulating material. In other words, the first insulating film 22in the first trench 20 below the position of 40% of the height of thefield plate electrode 24 does not contain the second insulatingmaterial.

When a reverse electric field is applied to the semiconductor device100, a high voltage is applied to the drain electrode 60. Meanwhile,since the field plate electrode 24 is often connected to the sourceelectrode 66 to be used as the same potential as the source electrode66, a voltage applied to the field plate electrode 24 is, for example, 0volt. In this case, since a high electric field is easily applied to abottom of the first trench 20, the bottom of the first trench 20 iseasily broken. When the second insulating material having a higherdielectric constant than the first insulating material is used for thebottom of the first trench 20 and the vicinity of the bottom of thefirst trench 20, the effective film thickness of an insulating film atthe bottom of the first trench 20 is smaller. Therefore, the bottom ofthe first trench 20 is more easily broken. Therefore, in thesemiconductor device 100 according to the present embodiment, the firstinsulating film 22 in the first trench 20 below the position of 40% ofthe height of the field plate electrode 24 contains only the firstinsulating material, or does not contain the second insulating material.

By further providing the third insulating film 30 provided between theside surface 24 c of the field plate electrode and the second insulatingfilm 26 a, a distance between the field plate electrode 24 and thesecond insulating film 26 can be controlled using the film thickness ofthe third insulating film 30. Therefore, the entire insulating filmthickness can be controlled independently of control of a difference inthe effective film thickness of the insulating film between the upperportion and lower portion in the first trench 20, and a breakdownvoltage can be easily controlled.

The second insulating film 26 has the first portion 26 a ₁ and thesecond portion 26 a ₂ provided on the first portion 26 a ₁, the secondportion 26 a ₂ having a larger film thickness than the first portion 26a ₁. This makes the effective film thickness of the insulating film inthe first trench 20 different between the vicinity of the first portion26 a ₁ and the vicinity of the second portion 26 a ₂. By using this, itis possible to control the electric field distribution so as to becloser to a rectangular shape when a reverse electric field is applied.

The film thickness W₂ of the second insulating film 26 between the sidesurface 24 c of the field plate electrode and the drift layer 12 ispreferably 30% or more of the film thickness W₁ of the first insulatingfilm 22 between the side surface 24 c of the field plate electrode andthe drift layer 12, where the second insulating film 26 is not provided.This is because when the film thickness W₂ of the second insulating film26 is less than 30% of the film thickness W₁ of the first insulatingfilm 22, the film thickness W₂ is too small, and an effect of providingthe second insulating film 26 cannot be sufficiently expected.

When silicon oxide is used as the first insulating material and siliconnitride is used as the second insulating material, the silicon oxide hasa compressive stress and the silicon nitride has a tensile stress.Therefore, the stresses cancel each other out. Therefore, it is possibleto suppress a disadvantage that a semiconductor substrate (wafer) isexcessively stressed to be warped.

The semiconductor device 100 according to the present embodiment canprovide a semiconductor device having a high breakdown voltage.

Second Embodiment

A semiconductor device 120 according to the present embodiment isdifferent from the first embodiment in that a field plate electrode 24has a third portion 24 e and a fourth portion 24 f provided on the thirdportion 24 e, the fourth portion 24 f having a larger film thicknessthan the third portion 24 e. Here, description of contents overlappingwith those of the first embodiment is omitted.

FIG. 16 is a schematic cross-sectional view of a main part of thesemiconductor device 120 according to the present embodiment. The fieldplate electrode 24 has the third portion 24 e and the fourth portion 24f provided on the third portion 24 e. The film thickness W₅ of thefourth portion 24 f is larger than the film thickness W₄ of the thirdportion 24 e. Here, the film thickness is, for example, a length in theXY plane.

The field plate electrode 44 has a portion 44 e and a portion 44 fprovided on the portion 44 e. The film thickness W₇ of the portion 44 fis larger than the film thickness WE of the portion 44 e. Here, the filmthickness is, for example, a length in the XY plane.

Since the field plate electrode 24 and the field plate electrode 44 eachhave portions having different film thicknesses, it is possible toprovide a semiconductor device having a higher breakdown voltage.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the semiconductor device describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the devices andmethods described herein may be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the inventions.

What is claimed is:
 1. A semiconductor device comprising: a firstelectrode; a first semiconductor layer of first conductivity typeprovided on the first electrode; a second semiconductor layer of firstconductivity type provided on the first semiconductor layer; a firstsemiconductor region of second conductivity type provided on the secondsemiconductor layer; a second semiconductor region of first conductivitytype provided on the first semiconductor region; a first insulating filmprovided in a trench reaching the second semiconductor layer from abovethe second semiconductor region via the second semiconductor region andthe first semiconductor region, the first insulating film containing afirst insulating material; a second electrode provided in the trench,the second electrode facing the second semiconductor layer via the firstinsulating film; a second insulating film provided between a position of40% of a height of the second electrode from a lower end of the secondelectrode and a position of an upper end of the second electrode, thesecond insulating film being provided between the side surface of thesecond electrode and a fifth insulating film provided between a sidesurface of the second electrode and the second semiconductor layer, thefifth insulating film containing the first insulating material, thesecond insulating film containing a second insulating material having ahigher dielectric constant than the first insulating material; a thirdelectrode provided above the second electrode, the first insulating filmand the second insulating film, the third electrode facing the firstsemiconductor region via a gate insulating film; an interlayerinsulating film provided on the third electrode; and a fourth electrodeprovided above the interlayer insulating film, wherein the firstinsulating film in the trench below the position of 40% of the height ofthe second electrode contains only the first insulating material.
 2. Asemiconductor device comprising: a first electrode; a firstsemiconductor layer of first conductivity type provided on the firstelectrode; a second semiconductor layer of first conductivity typeprovided on the first semiconductor layer; a first semiconductor regionof second conductivity type provided on the second semiconductor layer;a second semiconductor region of first conductivity type provided on thefirst semiconductor region; a first insulating film provided in a trenchreaching the second semiconductor layer from above the secondsemiconductor region via the second semiconductor region and the firstsemiconductor region, the first insulating film containing a firstinsulating material; a second electrode provided in the trench, thesecond electrode facing the second semiconductor layer via the firstinsulating film; a second insulating film provided between a position of40% of a height of the second electrode from a lower end of the secondelectrode and a position of an upper end of the second electrode, thesecond insulating film being provided between the side surface of thesecond electrode and a fifth insulating film provided between a sidesurface of the second electrode and the second semiconductor layer, thefifth insulating film containing the first insulating material, thesecond insulating film containing a second insulating material having ahigher dielectric constant than the first insulating material; a thirdelectrode provided above the second electrode, the first insulating filmand the second insulating film, the third electrode facing the firstsemiconductor region via a gate insulating film; an interlayerinsulating film provided on the third electrode; and a fourth electrodeprovided above the interlayer insulating film, wherein the firstinsulating film in the trench below the position of 40% of the height ofthe second electrode does not contain the second insulating material. 3.The semiconductor device according to claim 1, wherein a film thicknessof the second insulating film between a side surface of the secondelectrode and the second semiconductor layer is 30% or more of a filmthickness of the first insulating film where the second insulating filmis not provided, and the first insulating film is provided between theside surface of the second electrode and the second semiconductor layer.4. The semiconductor device according to claim 1, further comprising athird insulating film provided between a side surface of the secondelectrode and the second insulating film, the third insulating filmcontaining the first insulating material.
 5. The semiconductor deviceaccording to claim 1, wherein the second insulating film has a firstportion and a second portion provided on the first portion, the secondportion having a larger film thickness than the first portion in ahorizontal direction.
 6. The semiconductor device according to claim 1,wherein the second electrode has a third portion and a fourth portionprovided on the third portion, the fourth portion having a larger filmthickness than the third portion in a horizontal direction.
 7. Thesemiconductor device according to claim 1, wherein the first insulatingmaterial is silicon oxide, and the second insulating material is siliconnitride.